8253016: Box.Filler components should be unfocusable by default

Reviewed-by: prr, serb

src/java.desktop/share/classes/javax/swing/Box.java

@@ -316,6 +316,7 @@ public class Box extends JComponent implements Accessible {
             setMinimumSize(min);
             setPreferredSize(pref);
             setMaximumSize(max);
+            setFocusable(false);
         }
 
         /**

test/jdk/javax/swing/Box/TestBoxFiller.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/* @test
+   @bug 8253016
+   @key headful
+   @summary Verifies Box.Filler components should be unfocusable by default
+   @run main TestBoxFiller
+ */
+import java.awt.ContainerOrderFocusTraversalPolicy;
+import java.awt.KeyboardFocusManager;
+import java.beans.PropertyChangeEvent;
+import javax.swing.Box;
+import javax.swing.JFrame;
+import javax.swing.JTextField;
+import javax.swing.SwingUtilities;
+import java.awt.Robot;
+import java.awt.event.KeyEvent;
+
+public class TestBoxFiller
+{
+    private static JFrame frame;
+    private static void showFocusOwner(PropertyChangeEvent e)
+    {
+        Object c = e.getNewValue();
+        if (c instanceof Box.Filler) {
+            throw new RuntimeException("Box.Filler having focus");
+        }
+    }
+
+    public static void main(String[] args) throws Exception
+    {
+        try {
+            Robot robot = new Robot();
+            robot.setAutoDelay(100);
+            SwingUtilities.invokeAndWait(() -> {
+                frame = new JFrame();
+                KeyboardFocusManager m = KeyboardFocusManager.getCurrentKeyboardFocusManager();
+                m.addPropertyChangeListener("focusOwner", TestBoxFiller::showFocusOwner);
+
+                Box box = Box.createHorizontalBox();
+                JTextField tf1 = new JTextField("Test");
+                tf1.setColumns(40);
+                JTextField tf2 = new JTextField("Test");
+                tf2.setColumns(40);
+                box.add(tf1);
+                box.add(Box.createHorizontalStrut(20));
+                box.add(tf2);
+                frame.setContentPane(box);
+                frame.setFocusTraversalPolicy(new ContainerOrderFocusTraversalPolicy());
+                frame.pack();
+                frame.setVisible(true);
+                frame.setLocationRelativeTo(null);
+                tf1.requestFocusInWindow();
+            });
+            robot.waitForIdle();
+            robot.keyPress(KeyEvent.VK_TAB);
+            robot.keyRelease(KeyEvent.VK_TAB);
+        } finally {
+            if (frame != null) {
+                SwingUtilities.invokeAndWait(frame::dispose);
+            }
+        }
+    }
+}